An off-duty firefighter from Staten Island was slapped with drunk driving charges after a highway patrol officer spotted him swerving off a southbound West Shore Expressway exit ramp at 100 m.p.h. early this morning, police said.

The officer had just pulled another driver over onto the shoulder of the Muldoon Avenue exit ramp at around 1:40 a.m., when Robert Longo, 23, of Great Kills, came speeding down the Expressway in his 2002 Mitsubishi, abruptly switching from the left lane to the right lane, according to court documents.

Longo's car "came within 10 feet" of the police cruiser, which was parked with its lights flashing, police said.

The officer called ahead, and after a brief pursuit, Longo pulled over. At the time, police said his eyes were bloodshot and watery, his speech was slurred and he was unsteady on his feet.

Though he refused a Breathalyzer on the scene, a test at the 120th Precinct in St. George indicated Longo had a blood alcohol level of 0.14, or nearly twice the legal limit of .08.

Longo later admitted to drinking Thursday evening, police said.

He was charged with operating a motor vehicle while intoxicated, operating a motor vehicle while impaired and reckless driving.

According to New York motor vehicle records, Longo had his license suspended for 30 days in 2004 for excessive speeding.

Longo is a firefighter who works in Brooklyn's Engine 234. According to a fire department spokesperson, Longo's case is under review.